This invention relates to anti-backlash gearing systems constructed to take the shake and backlash out of mating gears in a power drive. Even though mating gears are constructed with the same tooth form, there will generally be clearance around a gear tooth in a mating gear tooth space, due to manufacturing tolerances and certain alignment inaccuracies. In machines of the metal chip removing type wherein heavy gear loads are seen, certain inertial effects may cause a driven gear to move around the driving gear tooth, thus creating a banging and hammering of gears. Further, on machines requiring a drive reversal, backlash clearance must be taken up first by driving gear movement, thus resulting in lost motion of the follower gear.
Many inventions in the prior art have addressed themselves to the problem of removing backlash from mating gears, and one popular system involves taking either the driven or driving gear and splitting it into two separate thinner gears which are countertorqued against one another to take up the tooth clearance. One common infirmity of these types of designs which will be discussed hereafter is that the countertorque forces are generally fixed, and may create undesirable tooth loads when the drive system is not engaged in a power cut. For example, in a rapid traverse movement, a high rate of pitch line velocity is seen, and it is often desirable to have a lessened countertorque force at that time.
Several prior art systems are contained in the following U.S. patents:
U.S. Pat. No. 2,081,644, Smythe, "Antirattling Gear", discloses a split gear system having the two gear segments on a common shaft counter rotated by hydraulic pressure ported to the inner surfaces of the gear bodies to remove backlash. One problem inherent with this drive system is that the gears are counterotated by what is in effect a hydraulic spring which may cause vibrational problems, and also presents a constant torque loading on the gear teeth.
U.S. Pat. No. 2,533,043, Price, "Backlash Eliminating Mechanism", discloses a spur gear drive train which is connected to a worm and worm wheel driven assembly through a pair of crossed helical gears. The pair of crossed helical gears are mounted on respective crossed shafts, and backlash is taken out of the system by driving the one shaft in a axial direction with a piston. The axial movement causes the helical gear teeth to act as a cam or wedge, thereby winding up the driving gear train, and loading all the backlash areas in one direction. The system has to be reversed when driving in the opposite direction, i.e., the actuating piston must be shifted in an opposite direction to again load all the backlash areas of the driving gear train in an opposite countertorque direction.
U.S. Pat. No. 2,655,050, Divette et al, "Antibacklash Gearing", discloses a machine drive wherein a pair of helical gears, relatively rotatable with respect to one another and located on a common shaft, are countertorqued by thrusting both gears in the same axial direction, reactively against a mating pair of helical gears which are relatively fixed with respect to one another. The opposite or counterrotation of the thrusted gears, causes respective drive gears connected to the rotatable helical gears, to be counterrotated against a driven member. Attention is drawn particularly to FIG. 3, and to col. 3, lines 54-64, "assuming motor 62 is not operating, it will be noted that the axial thrust on shaft 50 tends to rotate gears 49 and 48 in a direction to drive rack 46 and table 45 backwards whereas the same thrust tends to cause gears 52 and 47 to drive the rack and table in a forward direction. As a result, gears 47 and 48 pinch the rack teeth from opposite sides and provide a firm driving relation without backlash regardless of the direction of operation of motor 62." (Emphasis supplied). The thrust element in this system is a biasing spring. The spring tends to put a constant countertorque load on the driving gear teeth which remains in effect throughout all speeds of rotation.
U.S. Pat. No. 2,902,875, Finally et al, "Means for Reciprocating Elements of Machine Tools and the Like", discloses two complete and separate drive trains and motors connected to a driven element such as a machine rack. The driven gears are countertorqued against one another by their respective drive motors, one drive motor being the primary motor and the other motor being merely a drag load to take the shake out of the system. This system has a large number of gears in the drive trains, and the expense of plural drive motors.
U.S. Pat. No. 2,946,232, Jones, "Gear Drive", discloses two parallel drive trains connected to a common driving shaft and a single prime mover, wherein the shaft is split, allowing back-to-back shaft gears to be "phase-adjusted" by means of a ring gear which rotates a pair of planet pinions in mesh with a differential set of shaft gears. This type of assembly utilizes only one drive motor, but involves large numbers of gears in the system, and the ring gear provides a fixed adjustment which must be manually set.
U.S. Pat. No. 3,020,775, Musser, "Antibacklash Devices", discloses an apparatus for countertorquing a pair of coaxial gears, wherein one gear has a shaft extending through the other, hollow gear. The hollow gear has an internal helical gear at its opposite end, which receives an axially movable helical gear. The gears are, in effect, a "spiral spline". The helical gear has an internal spline which provides a direct torque couple to the shaft within the hollow gear. Axial adjustment of the helical gear by means of an axial adjusting screw causes a camming or rotation of the helical gears and shafts, thus countertorqueing the shaft mounted gear with the hollow gear. The adjustment is manually accomplished, and is preset until additional backlash is adjusted out of the system. The internal helical gear is a relatively expensive member to machine, and to closely mate with the external helical gear.
U.S. Pat. No. 3,151,494, Sedgwick, "Backlash Eliminator" discloses a machine table drive, wherein a pair of independent shaft mounted pinions are in mesh with a common bull gear and are countertorqued to take backlash out of the system. Countertorquing of the two gears is accomplished by training a timing belt around timing pulleys mounted to the shafts, and by impressing a midspan load on the belt by a transverse roller, which imparts a countertorque to the two pinions. The elastic belt used in this system results in a drive which is not stiff enough for many applications, and the drive tends to be more useful for unidirectional movement, that is where the power train is brought to one pinion driving against a "leading" flank. Reversing the direction of the drive causes the shaft which is connected to the drive to be relatively ineffective in back driving the bull gear, since the pinion is now biased to the trailing flank. The actual back drive would occur by virtue of the timing belt drive to the opposite shaft, which is a relatively weak drive system for that direction.
Applicant has obviated the difficulties inherent in these prior art devices by a novel anti-backlash drive system which is selectively supplied and adjustable as to preload. Further, the drive is able to be locked in a preload condition for continuous drive movement.